FLC OF AN MD83 ALLOW IT TO GET IN THE PROX OF MT RAINIER WHILE CLBING OUT FROM SEA DURING A NIGHT OP. PIC HAD OVERFLOWN THE REQUIRED RADIAL ON A MODIFIED DEP PROC WHILE ATTENDING TO ACFT SYS OPS.

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DEPARTING SEATTLE ON THE SUMMA 5 DEP SHORTLY AFTER XA00. INSTEAD OF PROCEEDING TO THE SEA 11 DME FIX AND TURNING L AS DEPICTED ON THE SID; DEP CTL GAVE US THE 130 DEG HDG RIGHT AT 5 DME TO AVOID ACR TFC WHICH WAS BEING VECTORED FOR THE APCH. IN ADDITION TO THE 130 DEG HDG; DEP CTL TOLD US TO INTERCEPT THE SEA 143 DEG RADIAL FROM THAT HDG. CLBING THROUGH APPROX 12000 FT MSL; I (PF) BECAME DISTR AS I WAS ATTEMPTING TO OPEN WING AND ENG ANTI-ICE VALVES AND TRYING TO HAND FLY AT THE SAME TIME. I LOOKED AT MY VOR NEEDLE JUST AS IT CTRED AND FLEW THROUGH THE SEA 143 DEG RADIAL BY NO MORE THAN 2-3 MI. NEEDLE DEFLECTION WAS APPROX 1/2 SCALE. BY THIS TIME; WE HAD SWITCHED TO ZSE FREQ 126.60 MHZ AND CTR ASKED US OUR HDG; OBVIOUSLY CONCERNED THAT WE WERE HEADED DIRECTLY TOWARD 14410 FT MT RAINIER AT AN ALT SLIGHTLY LESS THAN 13000 FT MSL. HE ASKED US IF WE HAD THE MOUNTAIN IN SIGHT (WE DID) AND TOLD US TO INTERCEPT THE SEA 143 DEG RADIAL. I DON'T BELIEVE TERRAIN SEPARATION WAS COMPROMISED BUT JUDGING FROM THE TONE IN THE CTLR'S VOICE; IT WAS ABOUT TO BE. THE ONLY TERRAIN THAT HIGH FOR MI AROUND AND I; THROUGH MY OWN MOMENTARY CARELESSNESS; ALLOWED THE SAFETY OF THE FLT TO BE COMPROMISED. ALTHOUGH THE INCIDENT OCCURRED IN THE MIDDLE OF THE NIGHT; I DON'T CONSIDER THAT THE LATE HR CONTRIBUTED TO THE INCIDENT. I WAS WELL RESTED; ALERT AND THE INCIDENT OCCURRED ONLY 15 MINS AFTER TKOF. MY NORMAL CYCLE WAS BROKEN (I FLY THIS TRIP EVERY 4 NIGHTS) BY THE BEING VECTORED OFF THE SID BY ATC AND MY CONCERN FOR TIMELY APPLICATION OF ANTI-ICE TO THE ENGS AND WINGS. IN THE FUTURE; I WILL PAY MORE ATTN TO ATC INSTRUCTIONS; PARTICULARLY WHEN AN AMENDMENT IS MADE TO A PROC SUCH AS A SID OR STAR WITH WHICH I AM VERY FAMILIAR.